In accordance with technological developments, various types of personal computers (PCS), such as desktops, towers and notebooks, have been developed and are being sold. The notebook PCS are made compactly and light, while taking portability and mobility for outdoor use into consideration.
One feature of a notebook PC it is "battery-operated type" that can be driven by an internally mounted battery power source. The battery is provided because a notebook PC is often employed in a mobile environment in which no commercially available power source is available. A battery power source used for a notebook PC is generally a "battery pack" in which a plurality of battery cells, such as Li-lon, NiCd or NiMH, are connected together. Such a battery pack weighs several hundreds of grams and outputs a voltage of from 4 to 12 V. While a battery pack of this type has a power capacity that lasts at most two to three computer operating hours, two to three hours are also required to charge it. The long charging hours constitute a serious problem for such a battery-operated apparatus, and a user must carry one or more spare battery packs in reserve. For this reason, for the notebook PC, a variety of power saving methods have been proposed that can extend the battery operating hours, even if only for a short period of time. The power saving function can be said to be another feature of the notebook PC.
Further, reflecting the ecological viewpoint, there has been an increased interest in saving on the power that is used to run desktop PCS, which are installed in offices and are driven by the endless power provided by commercially available power sources. The Environmental Preservation Association (EPA) announced a voluntary regulation called the "Energy Star Computer Program" in June of 1993, and recommended that the power consumption in the standby state be a reference level or lower (a driving power of 30 W or below, or 30% or less than is required for full operation of a CPU). Since the announcement, computer makers have competitively developed and manufactured products that comply with the proposed regulation.
The foundation or basis of the power saving design for various electric/electronic apparatuses, including PCS, is the halting of the power supply to devices and circuits that are not in use. A power saving design for a hard disk, which is one of the peripheral devices employed with a computer system, is disclosed in, for example, U.S. Pat. No. 4,933,785. The hard disk halts the supply of power to the internal electrical circuits in accordance with the state of the hard disk, i.e., in accordance with the time elapsed since the last disk access.
Recently, there has been rapid expansion of the multimedia function for PCS, i.e., a function for mixing computer data, and various other elements, such as motion and still pictures, and sound and characters, on a computer system. Since so-called multimedia data includes enormous files, such as for color image data, a multimedia PC must incorporate an external, portable storage device, such as a CD-ROM drive or a DVD (Digital Video Disc or Digital Versatile Disc) drive, that can hold such a large volume of data. Since the multimedia function is employed, for example, in the communication and broadcasting fields for TV conference presentations and color facsimiles, a device, such as a modem or a LAN card, must be mounted in a PC to provide a network connection, and this has led to the introduction of a PC card slot compatible ZV (Zoom Video) standard to support the moving picture function. A ZV port PC card transfers data directly to a video controller to implement the fast display of moving pictures.
A sound function, as well as the video function, is indispensable to the operation of a multimedia PC because one type of multimedia data handled by many of the peripheral devices associated with the multimedia function is audio data. To implement the PC sound function, an audio controller on a motherboard performs the mixing/volume adjustment and the D/A (Digital/Analog) conversion of audio signals supplied by the peripheral devices, an audio amplifier amplifies the resultant signals, and the amplified signals are either output at a loudspeaker, or are output to a line leading to an external audio device.
The power saving design for the PC will be explained hereinafter. As was previously described, the halting of the power supply to circuit modules that are not in use is essential for power saving; however, few power saving designs for the sound function has been put forward for the PC. This is partially because since there are many peripheral devices, such as CD-ROM drives, DVD drives, system beepers and modems, that are employed for the output of audio signals, and it is difficult to precisely detect the state when the sound function is not in use (or the state in which all the audio signals are muted).
An audio controller chip mounted on the motherboard of a PC usually has a power saving function that can be set or released by system software. That is, a specific bit of an I/O register in an audio controller is defined as a "power control flag," and when specific system software (e.g., a device driver for operating the audio controller) sets the flag to "1" in response to an I/O write access, the audio controller falls into the low-power consumption state in which the operation of the audio controller is disabled.
However, if the sound function of the system (e.g., an audio amplifier or a mixer circuit in an audio controller) is erroneously disabled while audio (CD-DA (Digital Audio)) data are being played by a CD-ROM drive, the audio data read from the disk are not output externally. This is because, even when the drive is changed from the muted state to the unmuted state, the PC can not detect such a state transition and, therefore, can not again enable the sound function in accordance with the operation of the drive. An external storage device having an audio function, such as a CD-ROM drive, can play audio data (in this case, perform line input to a PC) not only upon receipt of a host command from the PC, but also in response to the manipulation of a play button provided for the CD-ROM drive. When the play button is used, the PC software can not detect the muted state of the drive.
In other words, an audio output module, such as an audio controller, must be kept all the time in the active state in order to ensure normal operation of the PC sound function. Conventionally, even though an audio controller may have a power saving function that uses the "power control flag," the function is not actually utilized. Although in the active state, for example, the mixer circuit in the audio controller consumes several hundreds of mW of power, and such a consumption of power in the non-audio output state can not be ignored when a PC is battery operated.